1. Field of the Invention
The present invention relates to a method for continuously pelletizing glass making ingredients. More particularly, the invention relates to pelletizing glass making ingredients upon an inclined, rotating disc pelletizer usng concentrated liquid caustic as a binder and as a source of all or part of the alkali metal oxide content of the glass.
2. Brief Description of the Prior Art
For years it has been commercial practice to feed loose glass batch material to a glass making furnace. The batch ingredients which usually include sand, limestone, dolomite, soda ash and, optionally, salt cake, rouge, coal and other various ingredients, are first mixed in a dry mixer and then fed to the furnace, usually with a certain amount of cullet. Feeding of dry batch to the furnace in this manner generates dust, not only in the mixing and handling operations prior to the introduction into the furnace, but also afterwards within the furnace. Small particles of sand, soda ash and other ingredients are blown onto the walls of the furnace where they attack the refractory. In additon, particles of batch are blown through the stack and into the atmosphere where they can pose pullution problems.
To overcome these problems, there have been numerous teachings in the prior art to first pelletize the glass making ingredients before feeding them into a furnace. An over view of the prior art is disclosed in an article entitled "Pellets Cut Cost Improve Quality" by W. H. Engelleitner appearing in the March 1972 edition of The Glass Industry. This article discloses that it is known to pelletize glass batch making ingredients using various binders such as bentonite, clays, starches, lignin liquors and sodium silicate. However, the incorporation of certain binders into the pellets introduces a foreign material which may affect composition and quality of the resultant glass.
U.S. Pat. No. 3,081,180 to Krinov discloses that glass making ingredients can be pelletized by first putting the soda ash content of a glass batch into a water solution. The aqueous soda ash solution is then added to the remaining dry batch ingredients in a tumbling device where it acts as a binder to pelletize the glass batch.
U.S. Pat. No. 3,206,528 to Coombs et al discloses a method for pelletizing glass batch ingredients by radiantly heating a pelletizing mixture on the pelletizing pan to release moisture from hydrated constituents on the pan so as to partially dissolve portions of the mixture to form a cementing substance for coating of the granular particles of the mixture.
Canadian Pat. No. 745,666 to Blaha discloses a similar technique where part or all of the calcium and magnesium batch material is added in the form of burnt lime or dolomite. The addition of water to the batch ingredients on a rotating pan pelletizer provides a strong pellet chiefly through hydration of the burnt alkaline earths.
Another method for pelletizing glass batch ingredients is the substitution of liquid caustic as a partial or preferably as a total replacement of the soda ash content of the glass batch. Typical teachings in the prior art of such a process are disclosed in U.S. Pat. Nos. 3,542,534 to Yamamoto; 3,726,697 to Mod et al and South African Pat. Nos. 69-6971 to Summer and 70-4564 to Wilhelm and Gustav Eirich.
The Yamamoto patent discloses a two-step procedure for pelletizing glass batch material in which the dry batch is first mixed with caustic soda and water in a pug mill. The mixture is then transferred to a pan pelletizer where it is combined with additonal water to form pellets containing about 12 to 20 percent by weight water. This process has inherent disadvantages when compared to the method of the present invention. First of all, the dry batch and the caustic soda must be mixed with one another before being added to the pelletizer. This means that additional mixing equipment, which is in contact with the corrosive caustic soda, must be used. In addition, high water contents are required to make pellets by the Yamamoto process and the excess water must be removed by a two-step drying process before the pellets can be charged to a glass furnace or stored.
U.S. Pat. No. 3,726,697 to Mod et al discloses a two-step process for granulating glass batch using liquid alkali metal hydroxide as a total or partial replacement for the alkali fluxes contained in the batch. In the Mod et al patent, the alkali metal hydroxide substantially completely reacts with the alkaline earth carbonate contained in the batch to form a moist reaction mixture containing alkaline earth metal hydroxide, alkali metal carbonate and as little unreacted alkali metal hydroxide as possible. The moist mixture is then granulated into a particle and dried. The Mod et al process has a number of disadvantages associated with it. As in the Yamamoto process, the dry batch and caustic must be mixed with one another before granulation. This requires additional mixing equipment which must be contacted with hot corrosive alkali metal hydroxide.
South Africa Pat. No. 70-4564 discloses pelletizing with at least an 85 percent by weight caustic solution to form pellets containing at the most 5 percent, and preferably less than 3 percent by weight water. However, an 85 percent caustic solution is not commercially available and would require a temperature of greater than 450.degree. F. to insure that the caustic would be a liquid. Thus, added expense and severe caustic handling problems would be introduced into the pelletizing process.
South African Patent 69-6971 discloses a batch method for pelletizing glass batch in which the dry glass batch making ingredients are fed to a rotating drum so as to establish a falling curtain of such ingredients within the drum. Liquid caustic is then sprayed onto the falling curtain to form agglomerates of dry batch which then tumble in the bottom of the rotating drum to form pellets. Although pelletizing in a rotating drum forms spherical pellets the same as pelletizing on a rotating pan, there are significant differences between the two pelletizing apparatus. The main difference is the classifying effect of the pan. Normally, it is required to achieve as narrow pellet size distribution as possible. According to an article entitled "Possibilities of Influencing Pelletizing Pan Operation and Their Effect on the Properties of the Pelletized Material" by W. Pietsch, appearing in Aufbereitungs-Technik, 1966, April, pages 177-191, in the rotating drum, pellets of all sizes are produced since there is no means in the drum to segregate the various size pellets. Thus, it is necessary to screen with the rotating drum. The undersized pellets are returned to the drum and the amount in circulation can be 100 to 400 percent of the finished product. With the pelletizing pan, on the other hand, a classifying effect occurs during the movement of the particles on the pan. The nuclei (small pellets of agglomerating batch material) move in the vicinity of the pan (face), whereas the finished pellets move on top of the charge and are discharged over the edge of the pan. The size of the discharged pellets is quite uniform, being .+-.25 percent of a nominal size, so that there is no need to carry out classification. Furthermore, the pelletizing process on the open pan can easily be supervised and the size of the finished pellets can be influenced by feed rates, variations in rotational speed, inclination and height of the rim of the pan.
Besides the above disadvantages in the prior art on pelletizing glass batch material, the prior art has not sufficiently addressed itself to pelletizing glass batch material containing coarse sand.
It has been previously believed that since silica sand constitutes from 65 to 75 percent of the glass batch, it would be necessary to use a silica sand ground to a particle size much finer than that normally used in the glass industry. See the Mod et al and Yamamoto patents mentioned above. For example, silica sands used in both the flat and container glass industries east of the Mississippi River are ground so that 95 percent of the sand grains fall between 3 and 120 mesh screen sizes. However, prior pelletizing studies have indicated the need to use either silica flour, ground so that 100 percent of the material passes through 60, 140 or 200 mesh screens, or a fine whole grain sand product such as that mined from relatively unconsolidated deposits of fine grain in the southwest, particularly in Oklahoma. Therefore, a proposed pelletizing operation in eastern United States was faced with high freight costs to transport fine sand from Oklahoma or equally high production costs (additional grinding and classifying) to obtain a finely ground sand from current sources in the east. The ability to utilize the conventionally sized sand, very coarse by prior pelletizing standards, offers an obvious economic incentive. The present invention relates to both fine and coarse sand.